The currently urgent problem of finding more efficient welding techniques can be solved by cutting down the welding time by intensifying heating within a shorter welding period. The larger the section of welded pieces the more metal is saved.
Known in the art is a device for controlling the travel speed of welded pieces during flash-butt resistance welding (cf., for example, U.S. Pat. No. 3,335,257 Cl. 219-101, 1967) which comprises a hydraulic cylinder providing for a relative movement of welded pieces, a hydraulic follow-up control valve which controls the hydraulic cylinder and comprises a casing and a spring-loaded valve shaft, a reduction gear connected with the hydraulic valve shaft, a d.c. motor controlling the speed of convergence of welded pieces, and an electrical unit controlling the motor speed of rotation. The control unit comprises a cross-field rotary amplifier whose control winding is connected to a control voltage source (which controls the convergence speed of welded pieces). The control winding of the cross-field rotary amplifier is also connected to a welding current pickup. If the welding current exceeds a preset reference current level, current flowing through the control winding is the algebraic sum of the preset signal and that part of the feedback signal which exceeds the reference level.
The device is deficient in that it comprises a hydraulic control valve which is mechanically controlled and a reduction gear inevitably having gear clearances, making the response time of the device insufficiently long. The reverse movement command is executed only in 0.7-1 second in butt resistance welding machines equipped with such devices. The welding process cannot, therefore, be promptly controlled which is a serious drawback when dealing with welding of compact large-section pieces or thick-walled components.
Also known in the art is a device for controlling a flash-butt resistance welding machine (cf., for example, U.S. Pat. No. 3,555,237 Cl. B23K 11/04, 1971), comprising a welding transformer whose primary winding contains a workpiece voltage pickup connected, via a drive control unit, to an electrohydraulic drive actuating the moving machine.
The control unit feeds commands to an electrical coil of an electrohydraulic valve of the drive so that each half-period of the supply mains the direction and magnitude of current in the electrical coil are changed. The direction and volume of the hydraulic fluid passing through the electrohydraulic valve of the drive is, consequently, also changed and the head of the machine is correspondingly set into pulsating motion.
The control unit comprises a high-speed switching circuit connected to said electrical coil. The magnitude and direction of current in the electrical coil is the function of the voltage level of workpieces. The higher the voltage, the larger the "forward" period of the head movement within each half-period and the less the voltage the less the "forward" movement of the head and the more the "reverse" movement.
The high-speed switching circuit of the control unit comprises two transistors, the electrical coil being inserted between the collectors of said transistors in series with two potentiometers connected in parallel with two diodes. The base of one transistor is connected, via a resistor, to a collector of another transistor. In consequence, if the second transistor is non-conductive, the first transistor is necessarily rendered conductive and current from the power source passes through the electrical coil and the transistor to shift the shutter of the electrohydraulic valve so that the machine moves forward. Current passing through the coil can be adjusted by one of the potentiometers, the second potentiometer being shunted by a diode. The speed of movement can thus be regulated by changing the current.
The base of the second transistor is connected to an output of a Schmitt trigger circuit whose input receives the feedback voltage proportional to the welding voltage supplied from the welding transformer.
If the feedback voltage exceeds the operating threshold of the Schmitt trigger circuit, said circuit changes its state and the second transistor of the control circuit becomes conductive, the first transistor being rendered non-conductive. In consequence, the current in the coil is reversed and the machine movable plate starts moving "back". The speed of movement can be regulated by the potentiometer unshunted by the diode.
This device is capable of maintaining a stable flashing process since it responds to changes of the welding voltage in each half-period.
But it is deficient in that repeatability of heating of workpieces is difficult to achieve as the welding voltage varies within a very wide range. In addition, unavoidable variations of the mains voltage and the machine impedance in the process of operation of this control device bring about changes in the process. The permanent preset speed of movement of workpieces, brought either together or apart, results, considering above perturbations, in alterations of the flashing intensity and, consequently, in unstable heating affecting the quality of the welded joint.